The present invention relates to digital printers and in particular ink jet printers.
Ink jet printers are a well known and widely used form of printing. Ink is fed to an array of digitally controlled nozzles on a printhead. As the print head passes over the media, ink is ejected to produce an image on the media.
Printer performance depends on factors such as operating cost, print quality, operating speed and ease of use. The mass, frequency and velocity of individual ink drops ejected from the nozzles will affect these performance parameters.
Recently, the array of nozzles has been formed using micro electro mechanical systems (MEMS) technology, which have mechanical structures with sub-micron thicknesses. This allows the production of printheads that can rapidly eject ink droplets sized in the picoliter (xc3x9710xe2x88x9212 liter) range.
While the microscopic structures of these printheads can provide high speeds and good print quality at relatively low costs, their size makes the nozzles extremely fragile and vulnerable to damage from the slightest contact with fingers, dust or the media substrate. This can make the printheads impractical for many applications where a certain level of robustness is necessary. Furthermore, a damaged nozzle may fail to eject the ink being fed to it. As ink builds up and beads on the exterior of the nozzle, the ejection of ink from surrounding nozzles may be affected and/or the damaged nozzle will simply leak ink onto the substrate. Both situations are detrimental to print quality.
In other situations, a damaged nozzle may simply eject the ink droplets along a misdirected path. Obviously, this also detracts from print quality.
Accordingly, the present invention provides a printhead for an ink jet printer, the printhead including:
an array of nozzle assemblies for ejecting ink onto media to be printed; and
a nozzle guard covering the nozzle array, the nozzle guard having an array of apertures individually corresponding to each of the nozzle assemblies; wherein each of the apertures in the guard are sized and configured to prevent misdirected ink ejected from the nozzle assembly from reaching the media.
In this specification the term xe2x80x9cnozzle assemblyxe2x80x9d is to be understood as an assembly of elements defining, inter alia, an opening. It is not to be interpreted to be a reference to the opening itself.
Preferably, the apertures in the guard are passages with a lengthwise dimension that significantly exceeds the bore size in order to provide a collimator for each of the nozzles.
It will be appreciated that for the purposes of this invention, the cross section of the apertures may be any convenient shape and a reference to the bore size of the aperture is not an implied limitation to a circular cross section.
In a further preferred form, the printhead is adapted to detect an operational fault in any of the nozzle assemblies and stop supply of ink to the nozzle assemblies in which an operational fault is detected. In this form, the printhead may further include a control unit with a fault tolerance facility that adjusts the operation of other nozzle assemblies within the array to compensate for any damaged nozzle assemblies.
In these embodiments, it is desirable to provide a containment formation for isolating leaked or misdirected ink from at least one of the nozzle assemblies, from the remainder of the nozzle assemblies. In a particularly preferred form, each nozzle assembly in the array has a respective containment formation to isolate any leaked or misdirected ink from each individual nozzle assembly from the remainder of the nozzle assemblies.
In one form, each of the nozzle assemblies use a thermal bend actuator to eject droplets and a control unit adapted to sense the energy required to bend the actuator and compare it to the energy used by a correctly operating nozzle assembly in order to detect an operational fault. In a preferred embodiment, the nozzle has contacts positioned so that a circuit is closed when the bend actuator is at the limit of its travel during actuation so that the control unit can measure the power consumed and time taken in moving the actuator until the circuit closes to calculate the energy required. If the control unit senses an operational fault in the nozzle, it triggers the fault tolerance facility and stops any further supply of ink to the nozzle assembly.
The containment formation necessarily uses up a proportion of the surface area of the printhead, and this adversely affects the nozzle packing density. The extra printhead chip area required can add 20% to the costs of manufacturing the chip. However, in situations where the nozzle manufacture is unreliable, this will effectively lower the defect rate.
In a particularly preferred form, the nozzle guard is adapted to inhibit damaging contact with the nozzles. Furthermore it is advantageous if the nozzle guard is formed from silicon.
The nozzle guard may further include fluid inlet openings for directing fluid through passages in the guard, to inhibit the build up of foreign particles on the nozzle array.
The nozzle guard may include a support means for supporting the nozzle shield on the printhead. The support means may be integrally formed and comprise a pair of spaced support elements one being arranged at each end of the guard.
In this embodiment, the fluid inlet openings may be arranged in one of the support elements.
It will be appreciated that, when air is directed through the openings, over the nozzle array and out through the passages, the build up of foreign particles on the nozzle array is inhibited.
The fluid inlet openings may be arranged in the support element remote from a bond pad of the nozzle array.
The present invention maintains print quality by retaining misdirected ink ejected from damaged nozzle assemblies. The elongate passages through the guard act as collimators that can collect ink on their side walls. Furthermore, the guard protects the delicate nozzle structures from being touched or bumped against most other surfaces. By forming the shield from silicon, its coefficient of thermal expansion substantially matches that of the nozzle array. This will help to prevent the array of passages in the guard from falling out of register with the nozzle array. Using silicon also allows the shield to be accurately micro-machined using MEMS techniques. Furthermore, silicon is very strong and substantially non-deformable.